The purpose of this study is to gain a better understanding of the drug busulfan and how it is handled in the body after it is taken by mouth. The study objectives are as follows: (1) To evaluate whether a test dose of busulfan (0.25-0.5 mg/kg) administered prior to the scheduled high-dose busulfan regimen can accurately predict the therapeutic busulfan dose during the high-dose preparative regimen. (2) To assess the relationship between the pharmacokinetic parameters Cl0, Ke, Vd obtained from the test dose and from standard therapeutic drug monitoring of busulfan (following the first high-dose of the preparative regimen). (3) To assess the relationship between the pharmacokinetic parameters Cl0, Ke, Vd obtained from the thirteenth dose compared to the first dose.